Light-conducting glass structure having a peripheral light-absorbent layer

ABSTRACT

A transparent glass body having a light-focusing effect and a refractive index decreasing progressively from its centerline outward and containing readily reducible metal oxides in at least the outer peripheral part thereof is caused to contact a gas having a reducing effect thereby to reduce the metal oxides and form a light-absorbent layer of metal colloid in the outer peripheral part.

Xeo lc United State Kitano et al.

[ 51 Mar. 21, 1972 LIGHT-CONDUCTING GLASS STRUCTURE HAVING A PERIPHERALLIGHT-ABSORBENT LAYER inventors: Ichiro Kitano, Kobe-shi; Ken Koizumi,Itami-shi; Yoshlro Ikeda, Nishinomiya-shi; Hiroyoshi Matsumura,Osaka-shi, all of Japan Nippon Selfoc Kabushiki Kaisha (a/k/a NipponSelloc Co., Ltd.), Tokyo-to, Japan Filed: Sept. 29, 1969 Appl. No.1861,897

Assignee:

Foreign Application Priority Data Oct. 1, 1968 Japan ..43/71721 U.S. Cl..350/96 R, 350/175 GN, 65/30, 65/32, 65/2 Int. Cl. ..G02b 5/14 Field ofSearch ..350/96, 96 B, 96 WG, 175 ON, 350/276; 65/30, 32, 2

References Cited UNITED STATES PATENTS 5/1956 Parsons et al. ..350/276 X3,083,123 3/1963 Navias ..350/l75 GN X 3,320,114 5/1967 Schulz......3S0/96 X 3,434,774 3/1969 Miller ..350/96 WG X 3,486,808 12/1969Hamblen ..351/169 X 3,563,057 2/1971 Rosenbauer ..35l/169 X OTHERPUBLlCATlONS Miller Article in Bell System Technical Journal Vol. 44 No.9 Nov. 1965 pgs. 2017- 2030 cited.

Kawakami et al. Article in Proceedings of the IEEE Dec. I965, pp. 2148and 2149 cited.

Primary ExaminerDavid H. Rubin Alt0rney-Robert E. Burns and Emmanuel .1.Lobato [57] ABSTRACT A transparent glass body having a light-focusingeffect and a refractive index decreasing progressively from itscenterline outward and containing readily reducible metal oxides in atleast the outer peripheral part thereof is caused to contact a gashaving a reducing effect thereby to reduce the metal oxides and form alight-absorbent layer of metal colloid in the outer peripheral part,

1 Claims, 2 Drawing Figures Patented March 21, 1972 FIG.

FIG.2

BACKGROUND OF THE INVENTION This invention relates generally to glassstructures and more particularly it relates to new light-conductingglass structures each consisting, essentially, of a glass body having alightfocusing effect and a light-absorbent layer formed in the outerperipheral part ofthe glass body.

In accordance with the present invention, a transparent body having sucha focusing effect is one which has a refractive index distribution suchthat, in any cross section thereof transverse to the direction in whichlight is intended to ad vance therethrough. the refractive indexdecreases progressively outward from the center. When a light beam isprojected through the sections ofthis transparent body, the beam iscontinually refracted or curved toward the direction in which therefractive index increases. An incident light beam focused to anappropriate size is repeatedly refracted in this manner and is conductedthrough the interior of the transparent body. r

A light beam conducted in this manner by being repeatedly refractedwithout any expansion thereof is accompanied by very little phasevelocity lag at the light existing part of the transparent body and isadvantageous for multiplex light pulse communication. Preferably, therefractive index distribution in any cross section transverse to thedirection of advance of the light beam substantially satisfies thefollowing equation.

n is the refractive index at the center;

r is distance from the center;

n is the refractive index at a distance r from the center; and

a is a positive constant.

It is known that when this equation is satisfied, there is no phasevelocity lag of an appropriate light beam conducted through thetransparent body, which, at the same time, has the optical effect of aconvex lens.

An example of such a transparent body having a light-focusing effect isa so-called gas lens described in the Bell System Technical Journal,July l964, pp. l469-l479. Another example is a light-conducting glassfiber having the aforementioned refractive index gradient and having alight focusing effect, as proposed in the Proceedings of the I.E.E.E.,Vol. 53, Dec. 1965, pp. 2l482l49, in which a light-conducting glassfiber provided in its outer peripheral part with a light-absorbing layeris described.

A light-absorbing layer of this character is used for the purpose ofabsorbing and removing light which advances along the light conductionpath as it is reflected by the outer periphery of the conduction pathand light infiltrating into the conduction path from the outer peripherythereof. However, various difficulties have been encountered inproviding a lightabsorbing layer on such a light-conducting glass bodyhaving a self-focusing effect.

For example, one difficulty occurring in the process of coating theouter periphery of the light-conducting glass body with a moltenlight-absorbent glass at a high temperature is that, when thetemperature of the light-conducting glass body exceeds the softeningpoint thereof, migration of the constituents composing thelight-conducting body occurs therewithin, whereby the aforementioneddesirable refractive index gradient is disrupted, and deleteriouschanges in the characteristics of the glass body such as loss offocusing effect and deterioration of the resolution thereof as a lensoccur.

Even when this migration of the glass constituents is prevented by usinga glass of low melting point or an organic substance for thelight-absorbent layer thereby to cause the temperature of thelight-conducting body during the coating procedure to be below itssoftening point, strain is caused at the interface between thelight-conducting glass body and the light-absorbent layer by thedifference in the thermal expansion coefficients thereof, and lightwhich reaches this interface undergoes double refraction. Furthermore,the light reaching the interface is subjected to reflection by thedifference in the refractive indexes at the interface. The resultingrays of light thus advance through the interior of the light-conductingglass body, becoming light of different phase velocities, that is,noise, in the case of multiplex light pulse communication and loweringthe resolution of the image in the case ofimage conduction.

SUMMARY OF THE INVENTION It is an object of the present invention toconducting glass structure consisting, essentially, of a glass bodyhaving a light focusing effect provided in its outer peripheral partwith a light-absorbent layer and to provide a method for producing thesame, which structure and method are not accompanied by the abovedescribed difficulties.

According to the present invention, briefly summarized, there isprovided a light-conducting glass structure comprising an opticallytransparent glass body having a refractive index distribution asdescribed hereinabove and having a lightfocusing effect and alight-absorbent layer of metal colloid formed in the outer peripheralpart of the glass body by reducing at least one metal oxide therein.

The nature, principle, details, and utility of the invention will bemore clearly apparent from the following detailed description when readin conjunction with accompanying drawing, in which like parts aredesignated by like reference numerals.

provide a light- BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a cross section of an example ofa light-conducting glassstructure according to the invention; and

FIG. 2 is a graphical representation indicating the refractive indexdistribution in the radial direction over the cross section of the glassstructure shown in FIG. 1.

DETAILED DESCRIPTION A glass body having a refractive index distributionsuch that, in a cross section thereof transverse to the direction inwhich light is to advance within the body, the index continuouslydecreases outward from the center of the cross section can be producedby a process, for example, as described in detail in US. patentapplication Ser. No. 806,368 filed on Mar. l2, I969, titledLIGHT-CONDUCTING GLASS STRUCTURES AND PRODUCTION THEREOF now abandonedin favor of a continuation-in-part application Ser. No. 147,256 filed onMay 26, [971.

Such a process comprises the steps of causing a glass containing atleast one first cation constituting a modifying oxide of the glass tocontact a source ofa second cation constituting a modifying oxide as theglass is maintained at a temperature at which the first and secondcations can migrate within the glass in a manner to cause a greatersubstitution of the first ca tion in regions of the glass nearer to thesurface thereof by the second cation, this substitution therebyestablishing a refractive index distribution within the glass whereinthe refractive index varies progressively in directions transverse tothe intended path of advance oflight, whereby the direction oflightadvance is refracted toward the direction in which the refractive indexincreases.

In general, the refractive index of a substance has a relationship tothe molecular refraction and molecular volume characteristic of thatsubstance and increases with increase in the molecular refraction andwith decrease in the molecular volume. The molecular refraction isproportional to the polarizability of that substance. It is consideredthat the molecular refraction of a glass can be represented as anapproximation by the sum of the individual ion refractions. Accordingly,the qualitative effect of the presence of certain ions on the refractiveindex of a glass can be determined from a comparison of the values ofelectronic polarization per unit volume orthe values oflelectronicpolarisabilityl/(ion radius)" of the ions concerned.

In general, cations of larger ratios of electronic polarizability to(ion radius) within a glass have a tendency to contribute more greatlyto increase in refractive index. That is, this tendency in the case ofmonovalent cations is of the sequence: Tl Li K Na Rb. In the case ofdivalent cations, the relationship thereof in the order of degree ofcontribution toward increase in the glass refractive index is: Pb Ba CdSr Ca Zn Be Mg.

A glass body having a refractive index distribution such that the indexdecreases with continuity outward from the center in a cross sectionthereof transverse to the intended direction of advance of lighttherethrough can be produced, as described in the aforementionedcopending application, by causing a glass body containing certaincations constituting modifying oxides of the glass to contact a saltwhich contains cations having a lower value of (electron,polarisability)/(ion radius) than the first mentioned cations and,moreover, is capable of constituting modifying oxides of the glassthereby to cause a portion of the cations constituting modifying oxideofthe glass to be substituted by the cations within the salt.

In accordance with the present invention, a light-conducting structurecan be produced. in general, by heating a lightconducting glass bodyhaving a light-focusing effect and containing thallium oxide in theouter peripheral part thereofin a reducing environment at a temperaturebelow the softening point ofthe glass body thereby to reduce a partofthe thallium oxide and form a light-absorbent layer of colloidalthallium in the outer peripheral part of the glass body.

Examples of the reducing agent are hydrogen and carbon monoxide,hydrogen being preferably because it is more effective. The reducing gascan be employed in admixture with another reducing gas or in admixturewith a non-reducing gas.

Reduction of the metal oxide with the reducing agent proceeds from thesurface ofa glass structure in the treatment, and the metal oxide in thesurface area is reduced to form a metal colloid layer which islight-absorbent. The thickness of the layer can be controlled by theemployment of a proper combination of reducing conditions such astemperature, time, and type of the reducing agent for a givencomposition of a glass.

In order to indicate still more fully the nature and utility of theinvention, the following example of practice constituting a preferredembodiment ofthe invention is set forth, it being understood that thisexample is presented as illustrative only and that it is not intended tolimit the scope ofthe invention.

EXAMPLE A glass rod having a circular cross section ofa diameter of 1mm. and composed of 20 percent of T1 0, l percent of PhD, 14 percent ofNa O, and 56 percent of SiO (all percentages being by weight) wasimmersed for 48 hours in a potassium nitrate bath at 480 C., then washedwith warm water, and dried.

The glass rod thus treated had a refractive index of 1.57 at its centerand of 1.54 at its outer peripheral part, and the internal refractiveindex distribution was such as to be representable as an approximationby the equation n,=n,,( l-ar), where a=l 2 cm This glass rod having afocusing effect was subjected to a reducing treatment for 30 minutes inan electric furnace maintained at 450 C. and supplied thereinto withhydrogen gas at a flow-rate of 4 liters/min. and was then slowly cooled.The two ends of the resulting glass rod were ground to from planar endsurfaces perpendicular to the rod axis. The cross section of the rod tobe of the character indicated in FIG. I.

That is, within an annular region from the outer surface of the outerperipheral part 3 of the interior 2 of the glass rod I to a circleapproximately 30 microns inward from the outer surface, alight-absorbent layer of a black colloid 4 of Pb and TI was found tohave been formed. The interior 2, that is, the light-conduction part, ofthe glass rod was transparent, and the refractive index distribution wasfound to be unchanged relative to that prior to the reducing treatment.It was difiicult to distinguish the interface between the interior 2 andthe outer peripheral part 3, and the variation of the refractive indexat the interface was continuous as indicated in FIG. 2.

From other similar procedures, it was found that the thickness and thecolor tone of the light-absorbent layer differ depending on thetreatment time and temperature of the reducing treatment and differ alsowith the metal oxide reduced within the glass. It was found further thatoxides of silver, gold, copper, and nickel, as constituents, can alsoproduce colloids within the glass to form light-absorbent layers.

Thus, in the light-conducting glass structure according to theinvention, it is possible to form a light-absorbent layer at the outerperipheral part thereof without causing a migration of the glassconstituents of the light conducting part thereof. Moreover, therefractive index varies substantially continuously through thelight-conducting part and the light-absorbing absorbing layer, wherebyreflection does not occur at the interface therebetween. Accordingly,excellent signals can be transmitted particularly in multiplex lightpulse communication, and images or pictures of excellent resolutions canbe transmitted in conduction of pictures.

Furthermore, because of the existence of the metal colloid of thelight-absorbent layer at the outer peripheral part of thelight-conducting glass structure according to the invention, themigration of Na and TI within the glass is suppressed, whereby theweather resistance of the glass structure is excellent.

Particularly in a glass structure to which the refractive indexdistribution has been imparted by an ion-exchange process, therefractive index distribution at the outer peripheral part thereof tendsto deviate from that indicated by the equation set forth hereinbefore,Accordingly, a light-absorbent layer is formed in this outer peripheralpart to absorb light passing therethrough, and, therefore, thelight-conducting structure according to the invention is capable ofexhibiting high resolution when used in the conduction ofimages orpictures.

We claim:

I. A light-conducting glass structure comprising an opticallytransparent glass body of uniform cross section to define a rod or fiberconfiguration, said glass body containing thallium oxide and having adecreasing concentration of said thallium oxide, in cross section, fromthe central axis to the periphery, to provide a continuously decreasingrefractive index gradient substantially corresponding to the formulan,.=n,,( l-ar), where n is the refractive index at the center axis, r isthe distance from the central axis, n, is the refractive index atdistance r, and a is a positive constant; the thallium oxide in theperipheral area adjacent the outer surface being reduced by contact witha reducing agent to form an integral light absorbing layer of thalliummetal colloid extending inwardly from the peripheral surface.

